1. Field of the Invention
This invention relates to fuel assemblies for use in nuclear reactors and, more particularly, to locking techniques for the end fittings and control rod guide tubes in a nuclear fuel assembly, and the like.
2. Description of the Prior Art
To produce useful power from a nuclear reactor it is necessary to assemble fissionable material in a concentration that is sufficient to sustain a continuous sequence of neutron-induced fissions. Frequently, this concentration is attained by sealing uranium dioxide pellets in long, slender hollow rods. These rods, when filled with a charge of nuclear fuel and sealed at the ends, are called "fuel rods".
The fuel rods are arranged in a generally cylindrical array, or reactor core, to form the required concentration of fissionable material. In order to extract the heat generated in these fuel rods through the fission process, the fuel rods usually are spaced laterally from each other and water is pumped under pressure through the reactor core. The water absorbs the fission process heat and transfers this heat to secondary cooling water. The secondary cooling water rises into steam that is used to drive power generating turbine machinery.
In the reactor core, the radiation, pressures, temperatures and cooling water flow velocities create an environment that is quite hostile to the structural integrity of the reactor core. To cope with this environment, it has been customary to arrange the fuel rods that comprise the reactor core into a number of groups each of about two hundred fuel rods. These groups are frequently called fuel assemblies.
To enhance the structural integrity of each of the fuel assemblies and to stabilize the fuel rods in the assembly, it is common to mount the fuel rods between "end fittings" and to engage the mid-portions of each of the rods in the fuel assembly by means of cellular grid structures that are positioned at predetermined intervals along the lengths of the rods.
The structure of the fuel assembly, moreover, is not restricted to fuel rods, end fittings and grids. As a general rule one or more control rod guide tubes also are accommodated in the usual fuel assembly. Typically, to control the power generated in a nuclear reactor it is customary to add neutron absorbing materials to the reactor core. These materials have the effect of decreasing the fission activity within the core and thereby decreasing the power output from the reactor. As might be expected, there are a number of ways in which these neutron absorbing materials are introduced into the reactor core. Quite frequently, for example, the neutron absorbing materials are loaded into control rods. These control rods are received in hollow metal control rod guide tubes that extend through the length of the respective fuel element. In these circumstances, the depth of the penetration of the control rods into the associated fuel element determines, to some extent, the level of neutron fission activity and associated power output from the reactor core.
Some fuel assembly designs have a further use for the control rod guide tubes beyond aligning the individual control rods within the respective fuel assembly. Typically in this regard, the control rod guide tubes are often used to space the two end fittings from each other and, essentially, to clamp the fuel rods in proper relative position between these end fittings through enabling the end fittings to engage the extreme ends of the fuel rods.
This foregoing fuel assembly construction produces a rugged, sturdy structure that is able to cope with the forces that characterize a reactor core environment. There is, however, a somewhat countervailing need to provide a fuel assembly structure that can be assembled and dismantled with ease in order to reduce manufacturing costs, improve quality assurance and facilitate inspection and replacement. If it is realized that fuel assemblies, once having been made radioactive, must subsequently be manipulated behind shielding with remote handling equipment, the importance of the need for simple disassembly techniques becomes immediately apparent.
In this respect, the typical fuel element is dismounted by unthreading nuts that connect the control rod guide tubes to the end fittings, releasing one or more springs and, in general, taking the entire fuel assembly apart piece-by-piece. Not only is this a very laborious and expensive practice but it also introduces the possibility that one or more of the smaller fittings might go astray, leading to further lost time and expense, or damage if not discovered.
Thus, there is a clear need for an improved fuel assembly that will, to a large extent, overcome many of these inadequacies that have characterized the prior art.